public class Solution2 {
    public String longestPalindrome(String s) {
        int n = s.length();
        int begin = 0;
        int len = 0;
        for(int i = 0; i < n; i++) {
            // 先扩展奇数长度
            int left = i;
            int right = i;
            while(left >= 0 && right < n && s.charAt(left) == s.charAt(right)) {
                left--;
                right++;
            }
            if(right - left - 1 > len) {
                begin = left + 1;
                len = right - left - 1;
            }

            // 扩展偶数长度
            left = i;
            right = i + 1;
            while(left >= 0 && right < n && s.charAt(left) == s.charAt(right)) {
                left--;
                right++;
            }
            if(right - left - 1 > len) {
                begin = left + 1;
                len = right - left - 1;
            }
        }

        return s.substring(begin, begin + len);
    }
}
